1. Field of the Invention
The present invention relates to hexagonal strontium ferrite magnetic powder and a method of manufacturing the same. More particularly, the present invention relates to hexagonal strontium ferrite magnetic powder that is suitable for use as a magnetic material in magnetic recording media for high-density recording.
The present invention further relates to a magnetic recording medium comprising the above hexagonal strontium ferrite magnetic powder in a magnetic layer, and to a method of manufacturing the same.
2. Discussion of the Background
Hexagonal ferrite powder is widely employed as magnetic powder in magnetic recording. It has a coercive force that is high enough for use as a permanent magnetic material. Its magnetic anisotropy, which is responsible for the coercive force, is derived from the crystalline structure and thus can ensure a high coercive force even when the size of the particles is reduced. Further, a magnetic recording medium with a magnetic layer in which hexagonal ferrite magnetic powder is employed will have a high density characteristic due to the vertical component. Thus, hexagonal ferrite magnetic powder is suited to achieving high densities. Known methods of manufacturing hexagonal ferrite magnetic particles include the glass crystallization method, the hydrothermal synthesis method, and the coprecipitation method. From the perspective of obtaining a magnetic powder having the microparticle suitability and single particle dispersion suitability that are desirable in a magnetic recording medium, the glass crystallization method is a good method of manufacturing the hexagonal ferrite used in magnetic recording media. Thus, various methods of manufacturing hexagonal ferrite magnetic powder by the glass crystallization method have been examined (for example, see Japanese Examined Patent Publication (KOKOKU) Showa No. 60-15575 or English language family member U.S. Pat. No. 4,341,648, and Japanese Unexamined Patent Publication (KOKAI) Showa No. 56-169128 or English language family member U.S. Pat. No. 4,569,775, which are expressly incorporated herein by reference in their entirety).
In recent years, ever higher levels of recording density have been achieved in the field of magnetic recording. Magnetic tapes employing hexagonal barium ferrite magnetic powder achieving a surface recording density of 29.5 bpsi have been announced. Achieving still higher levels of high-density recording will require employing microparticulate hexagonal ferrite magnetic particles to reduce noise.
However, when the size of hexagonal ferrite magnetic particles is reduced, the energy for maintaining the direction of magnetization of the magnetic particles (the magnetic energy) tends to become inadequate to counter thermal energy, and thermal fluctuation ends up compromising the retention of recording. The phenomenon of magnetic energy being overcome by thermal energy, thereby compromising recording, can no longer be ignored. This point can be described as follows. “KuV/kT” is a known index of the thermal stability of magnetization. Ku is the anisotropy constant of a magnetic powder, V is the particle volume (activation volume), k is the Boltzmann constant, and T is absolute temperature. Increasing the magnetic energy KuV relative to the thermal energy kT can inhibit the effect of thermal fluctuation. However, the particle volume V, that is, the size of the particles in the magnetic material, should be small to reduce medium noise, as set forth above. Since the magnetic energy is the product of Ku multiplied by V, as stated above, it suffices to increase Ku to increase the magnetization energy when K is in the low range. However, Ku is related to the anisotropic magnetic field by HK=2Ku/Ms. When Ku is increased without changing Ms, HK also increases. The anisotropic magnetic field HK (is the magnetic field strength that is required to achieve saturation magnetization in the direction of the hard magnetization axis. When HK increases, the reversal of magnetization by the magnetic head tends not to occur, recording (the writing of information) becomes difficult, and reproduction output drops. That is, the higher the Ku of a magnetic particle, the more difficult the writing of information becomes.
As set forth in the above description, it is extremely difficult to satisfy all three characteristics of high-density recording, thermal stability, and ease of writing. This is known as the magnetic recording trilemma. It will be a major issue in the future as advances are made to still higher density levels of recording. In magnetic recording, barium ferrite is widely employed as a hexagonal ferrite magnetic particle. However, strontium ferrite is known to have a higher Ku and σs than barium ferrite. In this context, since HK=2Ku/Ms and Ms=σs×ρ (ρ: specific gravity), by lowering HK while raising Ku to resolve the trilemma, strontium ferrite is an advantageous magnetic material.
However, in conventional techniques including the above publications, the hexagonal ferrite magnetic particles that are actually produced and used as magnetic powder in magnetic recording are all barium ferrite. Investigation conducted by the present inventors has revealed no instance of the use of strontium ferrite. One reason for this could be difficulty in reducing the size of strontium ferrite particles. In this regard, the present inventors thought that since the crystallization temperature of strontium ferrite is about several 10° C. higher than that of barium ferrite, it would tend to promote nucleus growth and produce coarse crystals in the process of crystallization by the glass crystallization method. Conversion from an irregular amorphous structure to a regular crystalline structure requires that strontium ferrite structural elements migrate by diffusion. However, due to the difference in the melting points of SrO (2,430° C.) and BaO (1,923° C.), the temperature of crystallization of strontium ferrite is presumed to be higher than that of barium ferrite. Further, strontium ferrite tends to have a broader particle size distribution than barium ferrite. Although the reason for this is unclear, it has impeded the use of strontium ferrite as a magnetic powder in magnetic recording.